1. Field of the Invention
The invention relates to an electrostatic discharge (hereinafter abbreviated as ESD) protection semiconductor device.
2. Description of the Prior Art
With the advancement of technology, the development of semiconductor process is ongoing. A modern chip is therefore allowed to have a plurality of various electronic circuits configured within. For example, the integrated circuits (ICs) integrated in the chip(s) can be divided into core circuits and input/output (hereinafter abbreviated as I/O) circuits, and the core circuits and the I/O circuits are respectively driven by different power supply sources with different voltages. And for receiving the externally provided power, pads for core circuits and I/O circuits are required.
However, it is found that electrostatic charges are easily transferred to the inner circuits in the chip by those pads during processes such as manufacturing, testing, packaging, and delivering, etc. The electrostatic charges impact and damage the inner circuits in the chip, and this unwanted condition is named electrostatic discharge (ESD). As products based on ICs become more delicate, they also become more vulnerable to the impacts from external environment. And thus, it is assumed that ESD is a constant threat to the modern electronics. Models related to ESD tolerance are divided into human body model (HBM) and machine model (MM). For commercial IC products, the general ESD specification is required that IC products must pass these tests, for example, MM ESD tolerance greater than 100V. As a countermeasure against to the ESD issue, there have been proposed ESD protection circuits/devices. Typically, during a normal IC operation, the ESD protection device is turned off. However when an ESD event occurs, the ESD protection device must be quickly triggered, so that the ESD current is bypassed from the inner circuit. There is therefore a continuing need in the semiconductor processing art to develop an ESD protection device having lower trigger voltage which can be quickly turned on in order to render immediate protection to the inner circuit. That is, lower threshold voltage (Vt) for the ESD protection device is required. In the meantime, demand for improving device robustness is increased.